This invention relates generally to the field of rigid disc drives, and more particularly, but not by way of limitation, to an improved burnish head, which can be utilized to facilitate manufacture of rigid magnetic recording media with extremely smooth surface characteristics.
Disc drives of the type known as "Winchester" disc drives or rigid disc drives are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent to the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator housing opposite to the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator housing rotates, the heads are moved radially across the data tracks along an arcuate path.
As the physical size of disc drives has decreased historically, the physical size of many of the disc drive components has also decreased to accommodate this size reduction. Similarly, the density of the data recorded on the magnetic media has been greatly increased. In order to accomplish this increase in data density, significant improvements in both the recording heads and recording media have been made.
For instance, the first rigid disc drives used in personal computers had a data capacity of only 10 megabytes, and were in the format commonly referred to in the industry as the "full height, 51/4" format. Disc drives of the current generation typically have a data capacity of over a gigabyte (and frequently several gigabytes) in a 31/2" package which is only one fourth the size of the full height, 51/4" format or less. Even smaller standard physical disc drive package formats, such as 21/2" and 1.8", have been established. In order for these smaller envelope standards to gain market acceptance, even greater recording densities must be achieved.
The recording heads used in disc drives have evolved from monolithic inductive heads to composite inductive heads (without and with metal-in-gap technology) to thin-film heads fabricated using semi-conductor deposition techniques to the current generation of thin-film heads incorporating inductive write and magneto-resistive (MR) read elements. This technology path was necessitated by the need to continuously reduce the size of the gap in the head used to record and recover data, since such a gap size reduction was needed to reduce the size of the individual bit domain and allow greater recording density.
Since the reduction in gap size also meant that the head had to be closer to the recording medium, the quest for increased data density also lead to a parallel evolution in the technology of the recording medium. The earliest Winchester disc drives included discs coated with "particulate" recording layers. That is, small particles of ferrous oxide were suspended in a non-magnetic adhesive and applied to the disc substrate. With such discs, the size of the magnetic domain required to record a flux transition was clearly limited by the average size of the oxide particles and how closely these oxide particles were spaced within the adhesive matrix. The smoothness and flatness of the disc surface was also similarly limited. However, since the size of contemporary head gaps allowed data recording and retrieval with a head flying height of twelve microinches (0.000012 inches, 12.mu.") or greater, the surface characteristics of the discs were adequate for the times.
Disc drives of the current generation incorporate heads that fly at nominal heights of only about 1.0.mu.", and products currently under development will reduce this flying height to 0.5.mu." or less. Obviously, with nominal flying heights in this range, the surface characteristics of the disc medium must be much more closely controlled than was the case only a short time ago.
In current disc drive manufacturing environments, it is common to subject each disc to component level testing before it is assembled into a disc drive. One type of disc test is referred to as a "glide" test, which is used as a go/no-go test for surface defects or asperities, or excessive surface roughness. A glide test typically employs a precision spin stand and a specially configured glide test head including a piezo-electric sensing element, usually comprised of lead-zirconium-titanate (PbZrTi.sub.3), also commonly known as a "pzt glide test head". The glide test is performed with the pzt glide test head flown at approximately half the flying height at which the operational read/write head will fly in the finished disc drive product. For instance, if the disc being glide tested is intended for inclusion in a disc drive in which the operational heads will fly at 1.0.mu.", the glide test will typically be performed with the pzt glide test head flying at 0.5.mu.". If the glide test is completed without sensing any surface defects, then the disc is passed on the assumption that there will be no adverse effects on the operational heads and the discs during normal operation with a nominal head flying height twice that of the pzt glide test head flying height.
If, however, surface asperities or defects exist on the surface of the disc under test, the passage of the glide test head over the surface asperity will result in excitation of the glide test head, due to either direct contact between the pzt glide test head and the surface defect, or the disruption of the nominal hydrodynamic relationship between the rotating disc and the pzt test head. Current glide test head technology allows for the detection of media surface defects in the sub-microinch range.
When surface defects are detected on a disc, the disc is subjected to an additional manufacturing step called "burnishing". Burnishing is accomplished through the use of specially configured burnishing heads. The burnishing head is engaged with a rotating disc and contact between the burnishing head and surface defects results in mechanical removal of the surface defects. Following the burnishing process, the glide test is again performed. Current economic considerations dictate that any given disc will be subjected to the burnishing and glide test processes only a limited number of times, such as twice, before--with the continuing presence of surface defects--being finally rejected.
With the continuing trend in increased disc surface smoothness, the configuration of the burnishing head has undergone significant developmental changes, and, with the extremely smooth discs associated with current and future technology disc drives, two principal engineering challenges have arisen. Firstly, the burnishing height of the burnishing head must be more closely controlled, and secondly, the tribological interaction between the disc media and the burnishing head must be more closely controlled.
In order to properly burnish such a smooth surface, the slider of the burnish head must fly in close proximity to the disc surface and within an extremely close tolerance range. As the burnish head burnishes and smoothes the disc surface, both the media surface and the surface of the burnishing pads on the burnishing head approach and atomically smooth condition, and the interaction between two such surfaces results in increased static friction, or "stiction", due to adhesion. This condition can lead, in turn, to damage to the thin carbon overcoat layer on the disc surface, or even to scratching of the disc surface.
A need clearly exists, therefore, for a burnishing head configuration that performs burnishing to the sub-microinch level, which counteracts the tendency of extremely smooth surfaces to adhere to one another, and which optimizes the tribological relationships between the burnishing head and the discs being burnished.